Post on 22-Jun-2020
Accumulation Variability in the Antarctic Peninsula: The Role of Large-ScaleAtmospheric Oscillations and Their Interactions*
BRADLEY P. GOODWIN1
AND ELLEN MOSLEY-THOMPSON
Byrd Polar and Climate Research Center, and Department of Geography (Atmospheric Sciences Program),
The Ohio State University, Columbus, Ohio
AARON B. WILSON, STACY E. PORTER, AND M. ROXANA SIERRA-HERNANDEZ
Byrd Polar and Climate Research Center, The Ohio State University, Columbus, Ohio
(Manuscript received 15 May 2015, in final form 28 November 2015)
ABSTRACT
A new ice core drilled in 2010 to bedrock from the Bruce Plateau (BP) on the Antarctic Peninsula (AP)
provides a high temporal resolution record of environmental conditions in this region. The extremely high
annual accumulation rate at this site facilitates analysis of the relationships between annual net accumulationAn
on the BP and large-scale atmospheric oscillations. Over the last ;45 years, An on the BP has been positively
correlated with both the southern annular mode (SAM) and Southern Oscillation index (SOI). Extending this
analysis back to 1900 reveals that these relationships are not temporally stable, and they exhibit major shifts in
the late-1940s and the mid-1970s that are contemporaneous with phase changes in the Pacific decadal oscillation
(PDO). These varying multidecadal characteristics of the An–oscillation relationships are not apparent when
only data from the post-1970s era are employed. Analysis of the longer ice core record reveals that the influence
of the SAM onAn depends not only on the phase of the SAM and SOI but also on the phase of the PDO.When
the SAM’s influence on BP An is reduced, such as under negative PDO conditions, BP An is modulated by
variability in the tropical and subtropical atmosphere through its impacts on the strength and position of the
circumpolar westerlies in the AP region. These results demonstrate the importance of using longer-term ice
core–derived proxy records to test conventional views of atmospheric circulation variability in the AP region.
1. Introduction
The Antarctic Peninsula (AP) is a climatologically
complex region that includes ice-free ocean, sea ice, land
ice, and significant topographic relief within a relatively
small area (Fig. 1). Air temperatures have increased, par-
ticularly along the west coast since the 1950s (e.g.,;2.58C;King 1994), reflecting one of the strongest positive re-
gional trends recorded globally (Marshall et al. 2002;
Turner et al. 2005). Rapid warming observed over the AP
has been associated with the strengthening of the cir-
cumpolar westerly winds primarily driven by anthropo-
genic forcing (Marshall et al. 2006). Enhanced advection
of warmer maritime air masses over the orographic bar-
rier of the AP and the resulting foehn winds warm the
cooler continental climate on the east side (Orr et al.
2004; Marshall et al. 2006). Disintegration of ice shelves
has progressed southward (Scambos et al. 2004) from the
northern tip as predicted by Mercer (1978) to occur in
response to the anticipated warming of the planet due to
increasing greenhouse gas emissions.
However,many factors have been identified as drivers of
recent AP/West Antarctic climate change including trop-
ical variability (Ding et al. 2011; Schneider et al. 2012;
Ding and Steig 2013; ClemandFogt 2015), anomalous sea
ice concentrations (Ding and Steig 2013), and Amundsen
Sea low (ASL) variability (Fogt et al. 2012; Turner et al.
2013; Hosking et al. 2013; Clem and Fogt 2013). All of
these factors have demonstrated highly regional and
seasonal impacts on the climate of the AP and West
* Byrd Polar and Climate Research Center Contribution
Number 1443.1Current affiliation: Senior Service Fellow at the Agency for
Toxic Substances and Disease Registry.
Corresponding author address: Ellen Mosley-Thompson, Byrd
Polar and Climate Research Center, The Ohio State University,
108 Scott Hall, 1090 Carmack Rd., Columbus, OH 43210.
E-mail: thompson.4@osu.edu
1 APRIL 2016 GOODWIN ET AL . 2579
DOI: 10.1175/JCLI-D-15-0354.1
� 2016 American Meteorological Society
Antarctica. These processes are linked to large-scale
atmospheric oscillations, which are important de-
terminants of both weather and climate in this region.
One of the primary controls on southern high-latitude
climate is the southern annular mode (SAM) (e.g.,
Thompson and Wallace 2000), defined by Gong and
Wang (1999) as the difference in zonal mean sea level
pressure (MSLP) between 408 and 658S. The positive
(negative) phase of the SAM is marked by high (low)
pressure anomalies in the midlatitudes and low (high)
pressure anomalies across the high latitudes and Ant-
arctica, which result in a stronger (weaker) than normal
and contracted (relaxed) circumpolar westerly wind belt.
Both the sign and magnitude of the SAM exert control
on SouthernHemisphere (SH) climate variations including
sea ice extent and precipitation (Kang et al. 2011;
Simpkins et al. 2012). Since the 1970s, the SAM has been
steadily trending toward positive values, and over the last
decade it has beenmore consistently positive (Thompson
and Solomon 2002; Marshall 2003a; Thompson et al.
2011). Positive SAM values are correlated with colder
temperatures over most of Antarctica, with the excep-
tion of the AP, where warmer temperatures prevail
(Marshall 2002; Schneider et al. 2004).
El Niño–Southern Oscillation (ENSO) (Trenberth
1997), a global ocean–atmosphere phenomenon originat-
ing in the tropical Pacific basin, is associated with tele-
connections (linkages) tomany parts of the globe including
Antarctica (Turner 2004; Fogt and Bromwich 2006;
Gregory and Noone 2008; Stammerjohn et al. 2008; Fogt
et al. 2011). Physically, ENSO imparts a strong influence
on the climate of the AP through the South Pacific con-
vergence zone (SPCZ), an area of low-level convergence,
clouds, and precipitation that extends fromNewGuinea
southeast toward French Polynesia (308S, 1208W)
(Vincent 1994). Recent austral spring [September–
November (SON)] trends in MSLP in the southwest
Atlantic Ocean associated with ENSO variability (La
Niña) have been linked to warming temperatures across
the northwest AP (Clem and Fogt 2015).
The strength of the ENSO teleconnection to the high-
latitude South Pacific and resultant SAM–tropical sea
surface temperature (SST) relationship is modulated by
the sign and strength of individual SAM events (Fogt and
Bromwich 2006; L’Heureux and Thompson 2006; Fogt
et al. 2011; Clem and Fogt 2013) with preferences for in-
phase coupling between the phenomena (positive SAM/
La Niña and negative SAM/El Niño) (Gong et al. 2010,
2013; Fogt et al. 2011; Ding et al. 2012). The climate of the
AP is therefore influenced by this coupling between the
SAM and ENSO. While a positive SAM exhibits lower
pressure near the Antarctic continent and stronger high-
latitude circumpolar westerlies, the SPCZ is oriented
southwest of climatology during cool ENSO (La Niña)events (Vincent 1994). This generates a positive feedback
with poleward transient eddy momentum flux that in-
teracts with the polar front to increase cyclonic activity
in the South Pacific sector of the Antarctic coast (Chen
et al. 1996). Conversely, during negative SAM the cir-
cumpolar trough is weakened as the high-latitude storm
track relaxes to the north while warm ENSO (El Niño)events shift the SPCZ equatorward as well, directing
FIG. 1. Location of the sites in the AP discussed in the paper.
2580 JOURNAL OF CL IMATE VOLUME 29
cyclones away from the AP and toward South America
(Fogt 2007; Eichler and Gottschalck 2013). Thus,
the combination of the SAM and ENSO and their
associated modulation of SH storm tracks (Fogt et al.
2011; Schneider et al. 2012) influence accumulation on
the AP.
In addition to the SAM and ENSO, the Pacific decadal
oscillation (PDO) also influences the climate of the AP.
Often described as ‘‘ENSO like’’ decadal-scale variability
in the North Pacific (Zhang et al. 1997), Mantua et al.
(1997) coined the term ‘‘PDO’’ to describe an inter-
decadal ocean–atmosphere pattern of anomalous SSTs
and MSLP observed in the North Pacific that strongly
impacts salmon populations. Unlike ENSO, the most
distinctive oceanic–atmospheric footprint of the PDO is
located in the North Pacific Ocean, where the positive
phase denotes warm (cool) SST anomalies in the eastern
(western) Pacific (with opposite SST anomalies during
negative PDO conditions). In fact, the PDO has been
shown to respond to many factors including stochastic
atmospheric processes, SST anomaly reemergence in
subsequent winters, and ENSO through both the atmo-
spheric bridge (Rossby waves) and oceanic wave pro-
cesses (e.g., Alexander et al. 1999; Newman et al. 2003;
Schneider and Cornuelle 2005). The PDO has changed
polarity in 1925, 1947, and 1977 CE (Common Era;
henceforth all dates are in CE although not designated
as such) with the period prior to 1925 characterized as a
‘‘warm’’ PDOphase. The ‘‘cool’’ phase observed between
1947 and 1977 resulted in more prevalent La Niña–liketeleconnection patterns while the warm post-1977
phase enhanced the frequency of El Niño–like patterns.
In the late 1990s, the PDO began trending negative spe-
cifically during SON, which has been independently
linked to a deepening of the ASL, hence impacting
temperatures in West Antarctica, and to a lesser degree
in the AP (Clem and Fogt 2015).
The PDO shares a common North Pacific signature
with the interdecadal Pacific oscillation (IPO) (e.g., Folland
et al. 1999; Power et al. 1999), a phenomenon of SST
and atmospheric circulation variability shown to quasi-
independently impact the SPCZ (Folland et al. 2002)
and thus, SH climate. Many authors suggest that the
PDO and the IPO are relatively equivalent in describing
Pacific-wide ocean–atmosphere climate variability
(Folland et al. 2002; Deser et al. 2004; Dong and Dai
2015), but this remains an active area of research inquiry.
However, similar tropically driven variability associated
with the PDO and IPO has been demonstrated to drive
both NH and SH climate variability on interdecadal time
scales. Evidence strongly supports a northward displace-
ment of the SPCZ from the long-term mean during both
the warm phase of ENSO (Vincent 1994) and ENSO-like
(PDO/IPO) variability (Garreaud and Battisti 1999;
Folland et al. 2002). This coincides with the eastward shift
of warmer SSTs and deep convection in the central and
eastern tropical Pacific. This alters the atmospheric
heating anomalies that are dynamically tied to the gen-
eration of Rossby waves and teleconnections throughout
both the NH and SH (Sardeshmukh and Hoskins 1988;
Lee et al. 2009).
Therefore, the change in the location of the SPCZ is
a primary mechanism through which tropical climate
variability is transmitted to higher latitudes in the SH.
The direct impacts of shifting the SPCZ on SSTs, SLP,
temperature, and precipitation across the midlatitudes
and tropical land areas of the SH have been documented
(e.g., Andreoli and Kayano 2005; Dong and Dai 2015).
The impacts extend to higher latitudes; for example,
Garreaud and Battisti (1999) address directly the im-
pacts of ENSO-like variability on the Bellingshausen
Sea region. Clem and Renwick (2015) demonstrate that
spring temperature trends over West Antarctica and the
AP for the period 1979–2014 are also related to an in-
crease in deep tropical convection along the poleward
side of the SPCZ, resulting from low-level wind con-
vergence and increases in SLP in the eastern Pacific.
Thus, the climate of this region reflects complex in-
teractions among natural and anthropogenic forcing in-
cluding those associatedwith stratospheric ozone depletion/
recovery and increasing greenhouse gas concentrations
(Thompson and Solomon 2002). There is also strong
evidence for both regional (Silvestri and Vera 2009) and
continent-wide (Marshall et al. 2013) reversals in the
relationship between the SAM and Antarctic tempera-
tures that indicates that the SAM’s influence on the
climate of Antarctica is nonstationary in time. Their
results suggest a limitation on the ability to derive a
proxy for the SAM directly from a single Antarctic ice
core. The results reported here demonstrate that the
Antarctic climate and its relationship to the SAM and
ENSO appear to be interdependent on the PDO, which
provides additional links between the northern/tropical
Pacific Ocean and the Southern Ocean and hence the
AP. Changes to the atmospheric and oceanic tele-
connections during different phases of these indices im-
pact the climate of theAP,which provides an opportunity
to further examine these interactions. Fortunately, from a
continental perspective, the AP contains the greatest
abundance of climate records based on direct meteoro-
logical observations with some extending well into the
1940s and one (i.e., Orcadas) extending to the beginning
of the twentieth century (Turner et al. 2004). Two major
climatic variables, air temperature and precipitation, can
be reconstructed over long time periods from ice core–
derived proxies as well.
1 APRIL 2016 GOODWIN ET AL . 2581
There is abundant evidence from ice cores (Thompson
et al. 1994; Thomas et al. 2008, 2009; Abram et al. 2010,
2011; Mulvaney et al. 2012), model simulations (Hansen
et al. 1999;Dethloff et al. 2010), and reanalysis data (Sime
et al. 2009) of rapid climate changes in the AP. Here we
exploit a new annually resolved ice core record of accu-
mulation from the Bruce Plateau (BP) in the AP. Accu-
mulation represents an integrated climate signal of the
thermodynamic properties of temperature and moisture
and influences the chemical composition of snowfall that is
preserved as ice and used for paleoclimate reconstruction.
This longer-term record extends observational records and
provides an opportunity to better understand climate
variability in the AP over the last century. However, the
forces driving it vary spatially and temporally and lead to
complex interactions. Furthermore, while reanalysis and
model data have been used to draw seasonal links between
tropical variability and the AP in the post-1979 period, the
focus of this study is on the annual relationships discernible
from the BP core over the last century. Section 2 presents
the data and methods used in this paper while section 3
discusses the role of various atmospheric oscillators in
modulating the variability of accumulation (precipitation)
over the BP. Section 4 provides a discussion of these
linkages and highlights the importance of understanding
them and their interactions, while section 5 presents the
conclusions of this research.
2. Data and methods
a. Annually resolved ice core proxy data from theBruce Plateau
A new ice core from the AP, the first from the BP,
provides an excellent opportunity to deepen our un-
derstanding of climate variability in the region, espe-
cially along the northwestern coast (Fig. 1). This core,
henceforth the BP core, is 448.12m long and was drilled
in 2010 through the BP ice field [66.038S, 64.078W;
1975.5m above mean sea level (MSL)] to bedrock. The
west coast of the AP is dominated by westerly maritime
flow from the Southern Ocean and/or South Pacific
while the east coast is dominated by continental flow
from the Antarctic interior (Turner et al. 2002). The
result is much warmer temperatures and more pre-
cipitation along the west coast and colder, drier condi-
tions along the east coast. The crest of the AP acts as a
topographic divide separating the two regions. The BP
ice field feeds glaciers flowing into the Larsen Ice Shelf
to the east and into numerous bays, includingBarilari Bay
on the west. Very high accumulation along the west coast
of the AP results in a strong westward displacement of
the BP ice divide so that it is in close proximity to Barilari
Bay. The BP ice core was drilled slightly east (;2km) of
this topographic divide where the basal topography is
smoother. The extremely high accumulation rate ob-
served during the drilling campaign and later determined
from the analysis of the BP core indicates that the site
experiences predominantly westerly maritime flow and
thus is ideally situated to capture the precipitation (and its
constituents) reflecting climate variability over the Bel-
lingshausen Sea (Fig. 1). This annual net accumulation
[1.84m of water equivalent (w.e.) per year from 1900 to
2009] ensures a high-resolution proxy climate history. It is
essential that the BP ice core time scale be precise for the
period encompassed by this study (1900–2009) because
the annually resolved ice core–derived proxy climate var-
iables are compared with atmospheric and/or oceanic ob-
servations that are aggregated into annual averages.
The concentration of methanesulfonic acid (MSA)
measured by ion chromatography (Dionex ICS-3000)
has been shown at other Antarctic coastal ice core sites
to vary seasonally, with a summer maximum and winter
minimum (Curran and Jones 2000; Curran et al. 2003;
Preunkert et al. 2007; Abram et al. 2013). The season-
ality reflects changes in biological productivity over the
course of the year, which makes it an excellent chemical
species for the construction of an extremely robust time
scale for the BP core (Fig. 2). In addition, the elevated
concentrations of gross beta radioactivity (Fig. 2b) from
thermonuclear testing arrived in Antarctica in 1964/65
(Crozaz 1969; Pourchet et al. 1983) and provide addi-
tional confidence in the time scale. Annually averaged
ice core data are assumed to correspond approximately
to calendar years, but uncertainty in the timing of the
austral summer maxima, identified by the seasonal peak
concentration of MSA, results in an uncertainty of ap-
proximately two months in the break between years.
The reconstructed annual net accumulation An and
oxygen isotopic ratio (d18O) provide proxy-based his-
tories of precipitation and temperature, respectively.
The Dansgaard and Johnsen (1969) model was used to
reconstruct the original thicknesses of the annual layers
(i.e., An) at depth in the BP core. Analysis of d18O was
performed with a Picarro Cavity Ring-Down Spectrom-
eter, Model L2120-i (d18O precision: 0.05&). The BP An
and d18O records from 1900 to 2009 have a time scale
uncertainty of less than one year and are contained in the
upper 194.23m of the core, or roughly the upper half of
the BP ice sheet. The precise time scale for An coupled
with its high interannual variability (s 5 60.56m w.e.)
makes this an ideal site for exploring how large-scale
circulation patterns affect accumulation in this region.
b. Southern annular mode
Several authors have created SAM indices of vari-
ous lengths back in time, although results in this
2582 JOURNAL OF CL IMATE VOLUME 29
investigation will focus mainly on two: the Marshall
SAM index (Marshall 2003a,b) and the Fogt re-
constructed SAM index (Fogt 2009; Fogt et al. 2009; Jones
et al. 2009). The annual (January–December) Marshall
index (1957–2009) is calculated using the monthly values
provided byMarshall (2003b). The Fogt index (Fogt 2009)
extends from 1905 to 2005 with annual (December–
November) values constructed from the seasonal res-
olution. The Jones reconstruction (Jones et al. 2009;
provided by Julie Jones to Aaron Wilson on 18 April
2014) uses the first principal component of extra-
tropical sea level pressure (SLP) as the predictand,
while the Fogt reconstruction uses the Marshall index,
which is based on station SLP. Visbeck (2007, 2009)
reconstructed Antarctic SLP variability by assuming
atmospheric mass conservation between Antarctica
(data very scarce) and the subtropical latitudes (more
data available) and has been shown to be less reliable
than other reconstructions in all seasons except austral
summer (Jones et al. 2009). A recently reconstructed
SAM index (Abram et al. 2014a,b) uses a suite of proxy
records from South America and Antarctica plus a new
ice core–derived temperature record [inferred from
deuterium (dD)] recovered in 2008 from James Ross
Island (Fig. 1) on the eastern side of the northern tip of
the AP. Finally, the much shorter SAM record from the
National Oceanic and Atmospheric Administration
(NOAA) (Mo 2000; NOAA 2000) is included for
comparison over the very recent period.
c. Southern Oscillation index
The SouthernOscillation index (SOI), the atmospheric
component of ENSO, is measured as the difference in
SLP between Tahiti and Darwin (Ropelewski and Jones
1987; Allan et al. 1991; Können et al. 1998). SOI data,
available at monthly resolution (Climatic Research Unit
1987) from 1866 to the present, were annually averaged
on a calendar year basis for this comparison.
d. Pacific decadal oscillation
The PDO describes SSTs in the North Pacific Ocean
and shows significant decadal variability (Mantua et al.
1997). The PDO index is determined by the first prin-
cipal component of SST anomalies in the Pacific Ocean
north of 208N. Data for the PDO were obtained from
Mantua (1997) for the period between 1900 and 2009
at monthly resolution and averaged to annual resolution
on a calendar year basis.
e. TransPolar Index
The TransPolar Index (TPI) (Pittock 1980, 1984; Jones
et al. 1999) is a low-frequency movement in the phase of
FIG. 2. Seasonal variations in the concentration ofMSA used to date the BP ice core: (a) the initial 11 years contained in the upper 50m
of the core, (b) the section of the core containing the 1964 beta radioactivity horizon, and (c) the first 8 years of the twentieth century
contained in ;10m of core. Comparison of (a) and (c) illustrates the thinning of the annual layers with depth.
1 APRIL 2016 GOODWIN ET AL . 2583
wavenumber 1 around the SH, which is expressed as an
oscillation in troughing/ridging between New Zealand
and South America. Monthly data, calculated as the nor-
malized pressure difference between Hobart, Australia
(438S, 1478E) and Stanley, Falkland Islands (528S, 588W),
were obtained from the Climatic Research Unit (1980)
and averaged over the calendar years for 1905–2004.
f. Gridded meteorological data
Global SSTs are available from NOAA’s Extended
Reconstructed SST, version 3b (ERSST.v3b; Smith and
Reynolds 2002). The SSTs are based on in situ data and
are aggregated monthly from January 1854 to the pres-
ent on a 28 3 28 grid (Xue et al. 2003; Smith et al. 2008).
The monthly data were averaged over calendar years for
1900–2009 for comparison with the BP ice core accumu-
lation. Interpolated outgoing longwave radiation (OLR)
data are from NOAA’s Climate Data Record Program
on a 18 3 18 grid from satellite observations from 1979 to
2012 (Lee 2014). Monthly SLP data were acquired from
the National Centers for Environmental Prediction
(NCEP)–National Center for Atmospheric Research
(NCAR) reanalyses for 1979–2012 (Kalnay et al. 1996).
g. Station temperature data
Annual temperature data are provided by the Scientific
Committee on Antarctic Research (SCAR) Reference
Antarctic Data for Environmental Research (READER)
project for two stations, Faraday/Vernadsky and Rothera
(SCAR 2004; Turner et al. 2004). Temperature records
from Faraday/Vernadsky (65.48S, 64.48W; 11m MSL)
begin in 1947 and from Rothera (67.58S, 68.18W; 32m
MSL) begin in 1976.
h. Changepoint analysis
A key challenge in changepoint analysis is the ability
to detect multiple changes within a given time series or
sequence. The changepoints in the time series of the 11-yr
running correlations between An and the Fogt SAM index
were calculatedusing theRchangepoint package presented
by Killick and Eckley (2014). The pruned exact linear time
algorithm was used to identify times when a change oc-
curred in themeanof the 11-yr running correlationbetween
accumulation and the SAM index. The number of
changepoints identified is determined using the Bayesian
information criterion to prevent overfitting of the model.
3. Accumulation variability on the AP and the roleof large-scale atmospheric oscillators
Temperature and precipitation are typically used to
characterize regional climate variability and where these
records are short, as in the AP, longer proxy records of
temperature and precipitation are inferred from oxygen
and hydrogen isotopic ratios (d18O and dD) and An,
respectively. The annual net accumulation reflects the in-
tegrated contributions of precipitation, sublimation, wind
scouring, and redeposition. Accumulation rates measured
in AP ice cores have increased over the last century and
most dramatically since about 1950 (Thomas et al. 2008) in
concert with a contemporaneous increase in temperature.
Figures 3a and 3b confirm that the BP has experienced an
increase in annual net accumulation over the twentieth
century (0.102m w.e.decade21) with enhanced accumu-
lation (0.193m w.e.decade21) since 1950 that is concomi-
tant with warming temperatures at the two nearest
meteorological stations (Faraday/Vernadsky and Roth-
era). Interestingly, the d18O record reflects only a modest18O enrichment (0.065&decade21) over the twentieth
century with a larger warming trend (0.157&decade21)
since 1975 (Fig. 3c). This suggests that in addition to local
near-surface temperatures other processes, for example
those influenced by tropical climate variability, may also
exert control on the isotopic signature of the water vapor.
The recent increase in precipitation on the AP has
been linked to the increasing positive trend in the SAM
index, which enhances westerly winds and cyclonic ac-
tivity in this region. Fogt (2007) used outgoing longwave
radiation and atmospheric moisture measurements be-
tween 1971 and 2000 to identify the location of the
predominant SH storm tracks under different SAM and
ENSO conditions. His analysis showed that under pos-
itive (negative) SAM conditions, enhanced (reduced)
cyclonic activity in the southern high latitudes leads to
an increase (decrease) in atmospheric moisture, and
hence precipitation, in regions close to the Antarctic
coast. No analysis of the effects of the PDO on SH storm
tracks was included by Fogt (2007), likely because the
PDO was in a persistent positive phase for nearly the
entire analysis window (1971–2000).
The Marshall SAM index extends back to 1957 and is
considered the most robust of the available indices
(section 2b). Proxy records are generally calibrated to
this index, but this implies that their relationship is sta-
tionary over the entire reconstructed record. Examina-
tion of the available ice core proxy data and their
relationship to the Marshall SAM index reveals that the
strength of the relationship varies with the specific proxy
and the location of the ice core. For example, Abram
et al. (2014a) found a strong relationship between the
Marshall SAM index and dD from the James Ross Is-
land ice core (Fig. 1). Thomas et al. (2008) report a
dramatic increase in accumulation recorded in the Go-
mez ice core (Fig. 1) from the 1960s onward that is
correlated with the SAM. On the BP, An, rather than
d18O, is more strongly correlated with the SAM. This
2584 JOURNAL OF CL IMATE VOLUME 29
is demonstrated by comparing the annual values of An
and d18O from the BP ice core with the Marshall SAM
and Fogt SAM indices (Fig. 4) for the post-1970 period.
This period is identified by the year (1971) in which the
maximum running correlation is attained and after
which the correlation steadily declines. Figures 4a and
4b reveal a strong positive and statistically significant
relationship betweenAn on the BP and both theMarshall
and Fogt SAM indices from 1971 to 2009 (Marshall:
correlation coefficient r5 0.506,p, 0.001; Fogt: r5 0.511,
p 5 0.002). Figures 4c and 4d reveal a positive but
weaker relationship between d18O on the BP and both
the Marshall and Fogt SAM indices (Table 1). All
correlations are based on detrended data, and signifi-
cance is determined using a two-tailed Student’s t test.
These results suggest that the ice core–derived An record
from theBPmight provide a proxy for the sign and strength
of the SAM, particularly over longer (pre 1900) time
scales. However, extending the analysis to 1957 reveals a
weakening of the relationship between An and d18O to
both SAM indices (Figs. 4a,b and 4c,d, respectively).
This suggests that, prior to 1971, processes in addition to
the SAM were influencing both An and d18O. In-
vestigating this further reveals that the An–SAM re-
lationship changes sign between 1957 and 1970 (Marshall:
r 5 20.13, p 5 0.656; Fogt: r 5 20.20, p 5 0.489), which
accounts for the weaker r values for the post-1957 period.
These r values are not statistically significant and thus the
sign change could result by chance. However, we argue
that the marked change in the sign of the correlation be-
tween An and the SAM for the pre-1971 period warrants
further examination for two important reasons.
The first reason is related to the well-known transition
or shift in the mid-1970s of the PDO from a cool phase
to a warm phase. The cool phase (1947–77) exhibits a
La Niña–like teleconnection pattern with cooler SSTs in
the tropical eastern Pacific (Zhang et al. 1997). During
La Niña conditions the SPCZ shifts farther southwest
directing storms into the South Pacific, where poleward
eddy flux momentum may interact with the polar front
and intensify cyclonic activity along the South Pacific
sector of the Antarctic coast (Chen et al. 1996). The
second reason stems from the reliance on the full
(1957 onward) Marshall SAM record by studies in-
vestigating relationships between the SAM and other
proxy climate variables (Thomas et al. 2008; Abram et al.
2014a,b). For example, to construct a temperature-based
proxy SAM index, Abram et al. (2014a) used theMarshall
SAMindex from1957 to 1995 to calibrate eachproxy record
whose r value was then used to determine its weighted
contribution to the reconstructed proxy SAM record.
To further investigate the temporal stability of the
relationship betweenAn (Fig. 5a) and the SAM (Fig. 5b)
over the twentieth century, 11-yr running correlations
(Fig. 5e) were calculated using the detrended time series
ofAn and the Fogt SAM index (red). The relationship is
intriguing as it elucidates two rapid and dramatic changes
in the sign of the An–SAM relationship, one in the late
1940s and another in the mid-1970s. These transitions are
contemporaneous with the well-documented change in
the PDO from a warm phase to a cool phase in the North
Pacific and later back to awarmphase (Fig. 5c).Although
many (but not all) of the r values do not exceed 0.52
(required for p , 0.1), the rapidity of the transitions,
their timing relative to a well-documented change in
SSTs in the Pacific basin, and the sustained (multidecadal)
nature of the event all argue against its origination as a
random (chance) occurrence in the record. Although the
other available SAM records (Fig. 6a), including the
Abram et al. (2014a,b) proxy record, are not completely
independent, each contains a contemporaneous rapid
and multidecadal change in its correlation with An on the
BP between the late 1940s and the mid-1970s (Fig. 6b).
FIG. 3. (a) Annual net accumulation (m w.e.) from 1900 to 2009
reconstructed from the BP ice core. (b) Annual temperatures (and
trend) for the full records available from Faraday/Vernadsky Sta-
tion (red) and Rothera Station (black) along the northwestern
coast of theAP (Fig. 1). (c) Annual average d18O from 1900 to 2009
reconstructed from the BP ice core.
1 APRIL 2016 GOODWIN ET AL . 2585
ENSO and SAM have been shown to interact on
intraseasonal time scales (L’Heureux and Thompson
2006; Fogt et al. 2011) and ENSO teleconnections be-
tween the tropics and the South Pacific–Drake Passage
region have been demonstrated although the strength
of their interaction varies on decadal time scales (Fogt
and Bromwich 2006). The correlation (1905–2005)
between annual values of SOI (Fig. 5d) and An on the
BP (Fig. 5e, black line), performed identically to that
for the SAM, yields an r value of 0.13 (p 5 0.204),
weaker than that for the An–SAM relationship (r 50.279, p 5 0.005). Although the strength of the re-
lationship varies over time, it is nearly always positive.
However, the striking feature is the marked strength-
ening of the relationship from the late 1940s to the
early 1970s during which the r values consistently ex-
ceed 0.52 (p , 0.1). This is the same period during
which theAn–SAM relationship (red line) changes sign
and the PDO is in a sustained negative (cool) phase
(Fig. 5f). Figures 5 and 6 highlight the synchronicity of
these behavioral changes. These data suggest that the
relationship between BP An and the SAM/SOI differs
during periods of negative versus positive PDO con-
ditions, and they likely play stronger or weaker roles at
different times such that their interactions with the
SAM change over time.
To investigate further the influence of PDO variabil-
ity on the climate of the AP, spatial correlations were
calculated between BP An and global SSTs, with both
datasets detrended. Changepoint analysis (section 2h) of
the An–SAM 11-yr running correlations identified two
changepoints that break the time series into three in-
tervals (1900–49, 1950–73, and 1974–2009). The mean of
the 11-yr running correlations is 0.260 for the early
period (prior to 1950), 20.424 for the middle period
(1950–73), and 0.448 for the recent period (1974–2009).
FIG. 4. Correlations between (a),(b) annual net accumulation and (c),(d) annual d18O on the BP and the (top)Marshall SAM index and
(bottom) Fogt SAM index. The r values are shown for two time periods: the full record back to 1957 and a shorter interval back to 1971 that
approximates the transition from the PDOwarm to the PDO cold phase during which the correlations weaken. All correlations are based
on detrended data and are shown in Table 1.
TABLE 1. Correlations (p values) based on the detrended time series for the BP An and d18O and the Marshall (1957–2009) and Fogt
(1957–2005) SAM indices. Values in boldface indicate 95% significance.
Marshall–An Marshall–d18O Fogt–An Fogt–d18O
1971–2009(05) 0.506 (p , 0.001) 0.315 (p 5 0.051) 0.511 (p 5 0.002) 0.315 (p 5 0.066)
1957–2009(05) 0.319 (p 5 0.020) 0.213 (p 5 0.125) 0.274 (p 5 0.056) 0.211 (p 5 0.144)
2586 JOURNAL OF CL IMATE VOLUME 29
A two-tailed Student’s t test of the means of the three pe-
riods reveals that they have statistically significant differ-
ences (p, 0.001), indicating that the changepoint analysis
identified statistically significant shifts in the time series.
For the early period (1900–49), BPAn shows positive
and weak (insignificant) correlations to both the SAM
and SOI (Fig. 5e), and the PDO shifts from near neu-
tral (average: 0.06) to positive (average: 0.32) after
1925. Figure 7a reveals no significant tropical Pacific
influence over the AP during the early period. Negative
correlations exist between accumulation and SSTs in
the Atlantic Ocean and become positive east of the
AP, although the reliability of southern high-latitude
SSTs is subject to uncertainty. The Fogt SAM index was
generally negative at this time with a brief, strong pos-
itive excursion in the 1930s (Fig. 6a). Weaker westerlies
and a weaker ASL associated with a negative SAM
(Turner et al. 2013) would inhibit precipitation from the
FIG. 5. Annual values of (a) net accumulation on the BP, (b) the Fogt SAM index, (c) the
PDO, and (d) the SOI are shownwith (e) 11-yr running correlations between the BPAn and the
Fogt SAM index (red) and the SOI (black) calculated using detrended data and (f) the 11-yr
running mean of the PDO. Shading indicates the time between the changepoints, 1950 and
1973, calculated by changepoint analysis (section 2h), used in the correlation between BP An
and the Fogt SAM index.
1 APRIL 2016 GOODWIN ET AL . 2587
western sources reaching the AP. Thus, moisture sour-
ces east of the AP may be more dominant when tropical
forcing is weak and SAM is negative.
From 1950 to 1973, the PDO shifts to a strong negative
phase (average: 20.57), and the relationship between
BP An and the SAM (SOI) becomes distinctly negative
(more strongly positive) (Fig. 5e). The SST correlations
reveal less influence on BP An from local moisture
sources around theAP; however, SSTs in the tropics and
subtropics are strongly correlated with BP An (Fig. 7b).
Positive correlations dominate the SPCZ region, far
western Pacific, and central North Pacific. Negative cor-
relations are evident along the equatorial Pacific, along
the western coast of North America, and over the Indian
Ocean. These patterns reflect both ENSO and the PDO,
such that a La Niña event (positive SOI) accompanied
by a negative PDO would enhance accumulation at the
BP site.
After 1973, the relationship between BP An and the
SAM becomes positive and the SAM begins an increas-
ing trend to the present. The PDO is primarily positive
(average: 0.27), and the An–SOI relationship fluctuates
from negative around 1980 to weakly positive for the rest
of the period. In this later period, the tropical influence on
An is still present (Fig. 7c) but has shifted westward rel-
ative to the 1950–73 period. Negative correlations be-
tween BP An and SSTs are present around the continent
except adjacent to the AP, particularly along the west
coast. This supports the modern understanding of the
SAM such that when the SAM is positive and the cir-
cumpolar westerlies are enhanced, the continent is rather
isolated and colder. However, the enhanced westerlies
tend to deepen theASL, which has been demonstrated to
be congruent with the trend in the PDO during SON as
well (Clem and Fogt 2015), which increases northerly
flow over the AP (Turner et al. 2013). This warm, moist
northerly flow increases accumulation at the BP site. The
tropical connection is still present such that a La Niña,and hence a southward shift in the SPCZ, would also
serve to increase accumulation at the site. Again, SON
trends in La Niña show increasedMSLP in the southwest
Atlantic, which also increases northerly onshore flow
across the northwestern AP. Thus, in the future, if the
current trends continue such that the SAM increases and
FIG. 6. (a) Six SAM indices and (b) their 11-yr running correlation with BP An. All correlations are calculated using detrended data.
Shading indicates the time interval between the changepoints, 1950 and 1973, calculated by changepoint analysis (section 2h), used in the
correlation between BP An and the Fogt SAM index.
2588 JOURNAL OF CL IMATE VOLUME 29
the PDO shifts toward a negative phase, high accumula-
tion would be expected to continue at the BP site as well
as over much of the northern AP.
4. Discussion
a. Interactions during the modern era (post 1957)
Having demonstrated that the An–SAM transitions
are contemporaneous with phase changes in the PDO, a
physical mechanism is proposed here to account for
the changes. The SAM has shown an increasing trend
since the 1970s but has only been persistently positive
since the early 1990s. Under these persistently positive
SAM conditions, the circumpolar westerlies are strong
and located just north of Antarctica, thereby directing
storms toward the AP. BPAn also exhibits an increasing
trend (0.327m w.e. decade21) particularly since 1975
(Fig. 3a). During negative SAM conditions, the cir-
cumpolar storm track weakens considerably as the main
westerly track relaxes northward toward Australia and
New Zealand. These shifts in the circumpolar storm
track are associated with the anomalous meridional
FIG. 7. Correlations between annual detrended BP An and SSTs from ERSST.v3b for
(a) 1900–49, (b) 1950–73, and (c) 1974–2009. Positive (negative) correlations are shaded in red
(blue) according to significance levels.
1 APRIL 2016 GOODWIN ET AL . 2589
propagation of upper-level transient eddy momentum
and baroclinicity inherent in SAM variability (Kidston
et al. 2010; Fogt et al. 2011).
Since 1977, the PDO has beenmostly positive although
recently (post 1998) it has trended toward a negative
phase. The positive PDO exhibits more El Niño–likeconditions that shift the SPCZ to the northeast (in re-
sponse to contracted Hadley circulation and a stronger
subtropical jet stream), limiting poleward-propagating
synoptic eddy momentum fluxes that can increase cy-
clonic activity near the AP (Chen et al. 1996). However,
accumulation on the BP continued to increase during this
period under the influence of the increasing SAM. Since
1999, atmospheric circulation has been dominated by the
negative PDO phase wherein more La Niña–like condi-
tions shift the SPCZ to the southwest, increasing the
poleward synoptic eddy momentum fluxes (Vincent
1985). This shift, coupled with the strong circumpolar
westerlies during a positive SAM, explains the ample
precipitation on the AP since the turn of the twenty-first
century as both the westerlies and poleward transient
eddy momentum fluxes influence the region.
When both the SAM and the PDO are negative, the
storm track associated with the SAM is pushed north-
ward, inhibiting accumulation on the AP. Under this
scenario, AP accumulation becomes more dependent
on the poleward transient eddy momentum fluxes
from the SPCZ, which are heavily influenced by ENSO
(Fogt et al. 2011) and, by corollary, the PDO. This is
bolstered by the dramatic shift toward negative An–
SAM correlations and the concurrent strengthening of
the An–SOI relationship from the mid-1940s to mid-
1970s (Fig. 5e). Thus, the influence of tropical climate
variability on AP accumulation is stronger when the
circumpolar westerlies are less influential (i.e., weaker
and northward as under negative SAM conditions).
Given the uncertainties in reanalysis data for the SH
prior to 1979 when negative SAM and negative
PDO conditions were more prevalent, and the fact that
conditions with both negative PDO and negative SAM
have only occurred in two years since 1979, direct analysis
of storm tracks under this scenario is severely limited.
Atmospheric modeling of these conditions could provide
further insights into the interactions of tropically influ-
enced circumpolar storm tracks that help explain the re-
lationship between the SAM and BP accumulation.
To test whether the PDO influence on the AP is in-
dependent of tropical ENSO effects, linear regression
was used to remove the SOI from the PDO index. This
technique does not entirely remove the ENSO signal
from the PDO as their relationship is likely nonlinear,
and in this case only the atmospheric component (SOI)
is removed. Analysis of the PDO residuals (SOI linearly
removed) regressed onto gridded SLP and OLR fields
demonstrates its influence on the Southern Ocean and
Antarctic continent from 1979 to 2012 when reanalysis
data are most reliable in the SH. This time period was
divided into two subsets to represent the dominant
warm phase of the PDO (1979–98) and the recent shift
toward a cold phase (1999–2012).
The influence of a warm phase PDO on the SLP and
OLR is apparent in the NH but extremely limited in the
SH (Figs. 8a,b). Linearly removing the PDO signal from
the SOI time series isolates the ENSO-only tropical
teleconnection to the AP (Figs. 8c,d,g,h). The SOI has a
greater influence on the South Pacific than the PDO.
The ASL region shows a negative relationship with the
SOI (Fig. 8c) such that an El Niño event would increase
ASL pressure, which would reduce BP An. In addition,
during an El Niño event, convection would be situated
over the equator and across southern South America
(Fig. 8d). However, since 1999 the PDO has been pre-
dominantly negative (cold phase) under which these
relationships are substantially different. Contempora-
neously, the SLP in the Amundsen Sea region has been
decreasing (Clem and Fogt 2015) in response to a neg-
ative trend in the PDO, an increasing SOI, and an in-
creasing SAM. SLP regression indicates that a cold
PDOphase would accompany lower SLP around theAP
(Fig. 8e), although this relationship is not statistically
significant. However, a significant (95%) positive re-
lationship exists between the PDO and SLP in the
western Pacific near the origin of the SPCZ. This is ac-
companied by a significant positive relationship between
the PDO and OLR to the southwest of the long-term
SPCZ position (Fig. 8f), which reflects increased con-
vection and a southwest displacement of the SPCZ as-
sociated with a cold phase PDO. Thus the PDO acts on
the SH independently of the SOI, supporting the results
of Folland et al. (2002), who found that shifts in the
position of the SPCZ are attributable to both ENSO and
the IPO (a quasi-symmetric Pacific-wide manifestation
of the PDO). During this cold phase, the relationships
between the SOI and SLP (Fig. 8g) and OLR (Fig. 8h)
are spatially similar to the prior warm phase (Figs. 8c,d)
although somewhat enhanced. The SLP relationship is
enhanced especially in the ASL region. Likewise, the
negative relationship with OLR around the Antarctic
continent and southwest of the SPCZ indicates that a La
Niña event would accompany a southwest shift in the
SPCZ and increased convection extending toward the
AP (Fig. 8h). This period might be regarded to some
degree as an analog for the 1950–73 period when trop-
ical forcing appeared to exert more influence on BP An
(Fig. 7b). This analysis demonstrates that both the PDO
and the SOI independently influence the climate of the
2590 JOURNAL OF CL IMATE VOLUME 29
AP during this most recent era (1979–2012) for which
reliable data are available.
b. Reconciling differences in the early records (pre1957)
Prior to 1950, differences in the correlations between
various SAM reconstructions and BP An become more
evident. The SAM indices (Fig. 6a) compare best over
the post-1978 period when more and higher-quality
data are available. An analysis by Jones et al. (2009)
concluded that the station-based Marshall index best
represents the SAM for the post-1957 period, which
includes high-latitude station data. However, exploiting
the longer ice core record requires a longer SAM his-
tory, which leaves the Fogt, Jones, Visbeck, and the
newly developed Abram reconstructions. These indices
are less reliable in the first half of the twentieth century
due to data scarcity. Moreover, each reconstruction is
based on different approaches. The reader is referred to
the individual papers detailing the construction of each
FIG. 8. Regression analysis for (left) 1979–98 and (right) 1999–2012 of the PDO with the SOI linearly removed
onto (a),(e) SLP and (b),(f) OLR and of the SOI with the PDO linearly removed onto (c),(g) SLP and (d),(h) OLR.
Stippling indicates 95% significance, and themean position of the SPCZ (108–308S, 1508E–1208W) is represented by
the black line.
1 APRIL 2016 GOODWIN ET AL . 2591
index. However, some key comparisons should be
noted. Despite the different methods and locations of
data used, Fig. 6b clearly shows that all SAM indices
(excluding NOAA) exhibit marked transitions in their
correlations with BP An during the mid-1940s and mid-
1970s, concomitant with PDO transitions. This further
supports the PDO’s role in modifying the relationship
between the SAM and BP An.
The Abram SAM relationship with BPAn is similar in
nature to the other reconstructions from the mid-1960s
to the early-1980s and prior to 1920 (Fig. 6b). However,
the periodicity in the 11-yr running correlations with the
BP An is markedly different for the Abram SAM index
compared to the other longer records particularly during
the 1930s and 1990s. During the 1930s, the Abram SAM
correlations are muted (zero correlation) while the
other indices exhibit a more positive relationship with
BP An. Two explanations for this difference are put
forth byAbram et al. (2014a), one questioning the linear
detrending of the SAM and station observations of
MSLP used in the Fogt SAM index (Jones et al. 2009)
and the other noting the lack of high-latitude observa-
tions used in the Fogt SAM index. However, Jones et al.
(2009) show that some of the seasonal differences be-
tween Fogt and Jones during the 1930s (note the positive
peak evident in the annual Fogt time series as well;
Fig. 6a) are largely due to station anomalies that are
much stronger near New Zealand and weaker in South
America. Therefore, these differences are more re-
flective of a regional SLP anomaly pattern than a
hemispheric SAM and bear similarity to the TransPolar
Index. This raises the question: Can the TPI also play a
role in how the SAM indices, each created with different
records, compare with BP An?
Figure 9 shows 11-yr running means of the Fogt and
Abram SAM indices along with the TPI (Climatic
Research Unit 1980), all of which have been standard-
ized. Over the full period, the annual and 11-yr running
mean Fogt index is more strongly correlated (r 5 0.45,
p, 0.001 and r5 0.53, p, 0.001, respectively) with the
TPI than the Abram index (r 5 0.09, p 5 0.373 and r 50.23, p 5 0.029, respectively). This strong correlation
between the Fogt index and the TPI stems from their
close relationship during the period from 1920 to the
mid-1950s as well as post-1980 (Fig. 9). During this time,
the 11-yr running mean Abram index is significantly
different from the Fogt index and the TPI. However, the
Fogt and Abram indices are highly correlated from 1955
to 1979 (r 5 0.87, p , 0.001). During this time, both in-
dices are significantly anticorrelated with the TPI (Fogt:
r 5 20.53, p 5 0.007; Abram: r5 20.79, p , 0.001) and
theAn–SAM relationship as well as the PDO are notably
negative (Figs. 5e,f). This varying relationship between
different SAM indices and the TPI appears to be modu-
lated by the phase of the PDO.
This draws into question the stability of proxy-based
SAM reconstructions back in time, as different SAM
indices may be more or less reflective of a circumpolar-
natured SAM (i.e., Fogt index) compared to one thatmay
be more regional (i.e., Abram index). As it applies to the
BP An, the question becomes what is the main source of
the precipitation? As mentioned previously, the domi-
nant precipitation source for the BP is from the South
PacificOcean. The accumulation is likely tiedmore to the
regional SLP patterns that occur upstream from the
Drake Passage. The Fogt index captures these regional
SLP anomalies in the western Pacific that are part of the
hemispheric nature of the SAM, thus reflecting a differ-
ent relationship with An than the Abram index during
certain periods. Therefore, care must be taken to ensure
that the specific SAM index used reflects the climate
variability of the study area in question, as differences in
the regional versus hemispheric signals associated with
each SAM index may skew the conclusions drawn con-
cerning its relationship with various climate phenomena.
5. Conclusions
The relationship between BP An and large-scale at-
mospheric oscillations has not been temporally stable
over the last century. The relationship between BP An
and the SAM depends on the phase of both the SAM
and the PDO. Tropical Pacific climate variability in-
fluences the accumulation on the AP more strongly
under negative SAM conditions when the circumpolar
westerlies are less influential (i.e., weaker and posi-
tioned farther northward). The BP An–SOI relationship
is primarily positive but is stronger during negative
PDO and negative SAM conditions. As the SAM be-
comes more positive, the BP An–SAM relationship also
FIG. 9. The 11-yr running means of the Fogt (red) and Abram
(green) SAM indices as well as the TPI (black) for 1905–2004.
2592 JOURNAL OF CL IMATE VOLUME 29
becomes positive, and theBPAn–SOI relationshipweakens.
These multidecadal characteristics of the accumulation–
oscillation relationships are not apparent when data
only from the post-1970s era are analyzed.
Running correlations between BPAn and SAM show a
sharp transition from positive to negative values between
1950 and 1973. During this time, the SAM ranges from
negative to neutral, with one exception around the early
1960s when the SAM was briefly positive. The PDO was
also in a negative phase andLaNiña–like conditions weredominant. The reduced influence of the SAM on the
accumulation over the AP allows tropical climate var-
iability to exert a larger influence on accumulation. As
the running correlations between SAM and accumu-
lation weaken, the positive correlations between SOI
and accumulation increase. These results support findings
that the strength of the ENSO teleconnection to the
South Pacific (Amundsen Sea and Drake Passage re-
gions) is modulated by the SAM (Fogt et al. 2011), but
they also suggest that the phase of the PDO is a modu-
lating factor. More work is needed to understand the
mechanisms through which the PDO modulates ENSO
(Dong and Dai 2015), and modeling may help confirm
how the position of the SPCZ affects the upper-level
dynamics necessary for the generation of Rossby waves
and their propagation throughout the SH (Garreaud and
Battisti 1999; Clem and Renwick 2015).
This analysis has demonstrated the importance of
using this new ice core–derived accumulation record to
broaden the understanding of atmospheric circulation
variability over theAP. The conventional view of the role
of SAM and other climate oscillators has been derived
primarily from post-1957 data, but this view is challenged
by longer proxy records such as that from the BP ice core.
Understanding how these complex forcings interact to
control the transport of heat and mass (water vapor) and
how they change over time hinges upon the use of these
proxy records.
The SAM signal preserved in proxy records depends on
the location from which the record is retrieved. Ice core
sites around the AP exhibit sensitivity to the SAM differ-
ently (Thomas et al. 2008; Abram et al. 2014a), so indi-
vidual sites must be investigated to determine the
suitability of their records for SAM reconstructions. This
may prove challenging as observational records necessary
for calibration are limited in duration and spatial coverage.
The results of this study may be applied to locations in the
AP that are dominated bymaritimewesterly flow (western
coast of the AP) and are situated in latitudes with signifi-
cant SAM-induced variability in the strength of the cir-
cumpolar westerlies. Locations on the east coast (e.g.,
James Ross Island) or farther south (e.g., Gomez) have
been shown to have different relationships between
accumulation (or isotopes) and large-scale atmo-
spheric oscillations, most specifically the SAM. The
relationships described in this study will be easier to
identify at sites with higher accumulation rates that
allow clear identification of each year in the record. In
the early twentieth century, the SAM indices, as well
as the proxy-based reconstruction discussed, are not
consistent due to the different locations of the sites
whose data are incorporated in their respective models.
The relationships with accumulation on the BP dis-
cussed here suggest that other processes in addition to the
SAM (e.g., the PDO and SOI) likely play stronger and
weaker roles at different times such that their interactions
with the SAMare not stationary. Accumulation on the BP
is modulated by variability in the tropical and subtropical
atmosphere through its impact on the strength and posi-
tion of the circumpolar westerlies. If the processes and
their interactions governing the temporal variability of
accumulation on the BP during the twentieth century can
be better elucidated, then the longer (;500yr) annually
resolved accumulation history available from the BP ice
core might provide additional clues regarding the longer
histories of these atmospheric oscillators.
Acknowledgments. The authors thank members of
the BP field team [Victor Zagorodnov, Vladimir
Mikhalenko, Benjamin Vicencio, Roberto Filippi (de-
ceased), and Thai Verzoni], who spent 42 days with Ellen
Mosley-Thompson drilling the BP ice cores. We thank
Raytheon Polar Services and the British Antarctic
Survey who provided essential logistical support of
the field activities. Bradley P. Goodwin thanks The
Ohio State University’s Graduate School and De-
partment of Geography and the National Science
Foundation (NSF) for graduate student support. We
thank Julie Jones for personally sending us her
SAM reconstruction. Interpolated OLR and NCEP–
NCAR reanalyses data were provided by NOAA/
OAR/ESRL PSD from http://www.esrl.noaa.gov/psd/.
The field project, laboratory analyses, and partial
support for Bradley P. Goodwin were provided by NSF
Award ANT-0732655 to Ellen Mosley-Thompson as
part of NSF’s IPY LARISSA Project. The ice core data
used in this study are archived in the Global Change
Master Directory (http://gcmd.nasa.gov/getdif.htm?
LARISSA_0732655) and NOAA’s National Climate
Data Center/Paleoclimatology (http://www.ncdc.noaa.
gov/data-access/paleoclimatology-data).
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